Disclosure of the present invention is facilitated by U.S. Pat. No. 5,818,867 issued on Oct. 6, 1998 to Donald J. Rasmussen, which is hereby incorporated by reference.
Quasi-bandlimited minimum shift keying (QBL-MSK) pulse-shaping proposed by Frank Amoroso provides a significant improvement in modulator spectral efficiency over standard MSK when operated with a linear power amplifier. Since QBL-MSK is not a constant envelope modulation waveform, nonlinear Class C power amplifications results in regrowth in the spectrum sidelobe levels. Spectral efficiency is lost by this regrowth process. Amoroso has shown that QBL-MSK offers significant improvement in spectral efficiency over standard MSK even with sidelobe regrowth. QBL-MSK pulse-shaping has been applied to spread spectrum radios by D. J. Rasmussen using binary phase shift keying (BPSK) to convey data information. By using BPSK data modulation, the QBL-MSK waveform structure is unmodified. Therefore, the spectrum results generated by Amoroso can be directly applied to the spread spectrum signal. Spreading is easily applied to the BPSK data modulation by using an exclusive-or device. FIG. 1 shows the power spectrum for BPSK/QBL-MSK at 4.8 Mcps with 16 chips per symbol, using a 12 bit digital-to-analog converter (DAC) with nonlinear amplifications.
Quadrature phase shift keying (QPSK) data modulation is used to increase the data rate capability over binary phase shift keying BPSK data modulation. Data capability is increased by a factor of 2 using QPSK. QPSK data modulation does modify the spread spectrum QBL-MSK waveform. FIG. 2 shows the block diagram for the QPSK/QBL-MSK modulation. Details of the system shown in FIG. 2 is given in U.S. Pat. No. 5,818,867. The modulation waveform is structured to allow use of a serial demodulation approach. Because QPSK data modulation modifies the spread spectrum QBL-MSK waveform, Amoroso's nonlinear results are not valid for this waveform. This new waveform has conditions in which the RF envelope goes close to or equals zero.
A signal condition in which the RF envelope deviation is minimal is shown in FIGS. 3A, 3B and 3C. For this condition, the QPSK data modulation does not have a significant impact on the QBL-MSK spreading waveform. FIGS. 4A, 4B and 4C show a signal condition in which the RF envelope deviation is severe. For this condition the RF envelope goes to zero, which is undesirable for a near constant envelope modulation waveform, like QBL-MSK. These large RF envelope deviations result in increased sidelobe level regrowth for nonlinear Class C power amplification. Sidelobe regrowth can be significantly reduced by using a Class AB power amplifier and backing off on the input drive level. This approach gives improved spectral efficiency at a price of power efficiency.
For BPSK data modulation, the SQBL-MSK spreading signal is not impacted by the data modulation. For QPSK data modulation, the SQBL-MSK spreading signal is impacted by the data modulation at the symbol boundary conditions, when either a −0.5π (−90 degree) or 0.5π (90 degree) phase change between symbols is being made. Two different 90 degree phase changes associated with QPSK data modulation, in which the past QPSK symbol is at 0 degrees and the present QPSK symbol is at 90 degrees are examined to show two significantly different RF envelope effects. Minimal RF envelope deviation, as shown in FIGS. 3A, 3B and 3C, results when the I and Q signals do not go to zero at the same point in time. Severe RF envelope distortion, however, as shown in FIGS. 4A, 4B and 4C, results when both the I and Q signals go to zero at the same point in time, causing the RF envelope to go to zero. As clearly shown in FIGS. 4A, 4B and 4C, the near constant RF envelope performance of SQBL-MSK is not preserved. To preserve the near constant RF envelope performance of SQBL-MSK, a phase mapping process is performed to prevent the severe RF envelope deviation condition. The phase mapping process changes the phase trajectory only about the symbol boundary, when either a −90 or 90 degree phase change occurs. This mapping prevents the severe RF envelope condition shown in FIGS. 4A, 4B and 4C.
FIG. 5 shows a block diagram of a QPSK/QBL-MSK modulator with phase mapping module 10 added to prevent the RF envelope from going close to or equal to zero. A phase mapping operation by module 10 is performed between serial formatting module 24 and QBL pulse-shaping modules 27. A description of phase module 10 is provided in U.S. Pat. No. 5,818,867 and incorporated herein by reference.
FIG. 6 shows a top level block diagram of phase mapping module 10. This module compares the present and past data modulated chips of both the I and Q signals to determine if they are inverted. If the data modulated chips are inverted on both the I and Q signals, the RF envelope will either go to zero or close to it. To prevent this undesirable condition for the RF envelope, module 10 inverts both the I and Q signals.
FIG. 7 shows the resulting power spectrum for the QPSK/QBL-MSK waveform with phase mapping using a 12 bit DAC for nonlinear power amplification at a 4.8 Mcps chip rate with 8 chips per symbol. Comparing these results using phase mapping by module 10 to those for BPSK data modulation shown in FIG. 1, the spectral regrowth is only slightly higher than that obtained for the BPSK data modulation.
The phase mapping shown in FIG. 6, by inverting the modulated chips, improves the power spectrum, but results in a reduction in the spreading gain by two chips. Two chips of processing gain are lost, since the inverted chip reduces the number for chip agreements by one with the additional 1 chip degradation introduced by inverted chip contribution to the symbol accumulation. For spreading ratios greater than 32 chips per symbol, the process gain degradation for the severe RF envelope condition is less than 0.3 dB. With increasing processing gain, this loss becomes smaller. For a 16 chips/symbol system this degradation is less than 0.6 dB. An 8 chips/symbol system this degradation is approximately 1.25 dB. For lower spreading gain of 8 chips per symbol, the degradation is greater than 1 dB. It is desirable to reduce this loss to less than 1 dB, because a lower spreading gain signal may then be used to increase data rate, which typically requires a higher operational Es/No to achieve acceptable bit error rate (BER) performance. To reduce this degradation, a mapping that does not invert both chips is required, and is provided by the present invention.